Meltblown products are a particular type of nonwoven material consisting of random collection of thermoplastic microsized fibers ranging in typical average diameters from about 1 to 15 microns. These products are available in the form of webs, cylinders, or rovings and make particularly good filters because of the small pore size in the randomly collected fibers.
Cigarette filters made from meltblown nonwovens, however, have not been particularly successful. It has been difficult to produce the nonwovens in the small cylindrical shape necessary for cigarette filters with the desired physical characteristics such as filterability and hardness. Filter production using meltblown web is presently limited by the small lengths of the web which can be packaged for a batch. Also rods from meltblown web are soft and have end recess.
U.S. Pat. No. 3,595,245 discloses a polypropylene meltblown cigarette filter. This filter is in the form of a roving produced by a circular meltblowing die. The circular die is expensive and difficult to operate. Moreover, they have not consistently produced filters of the quality necessary for cigarette filters. For these reasons, the cigarette filters of meltblown rovings manufactured from circular dies have not received notable commercial use.
There have been efforts to produce cigarette filters from nonwoven materials in addition to meltblown rovings. U.S. Pat. No. 3,346,682 discloses a cigarette filter made from a thin polymer sheet. The sheet is calendered and slit into tapes; each tape then is bulked and collected in a form suitable for cigarette filters.
U.S. Pat. No. 3,888,610 discloses a method of forming nonwoven rovings useful as cigarette filters. These rovings are produced from individual nozzles.
U.S. Pat. No. 4,059,121 discloses a small disc section of a filter packed with a mass of nonwoven polymeric fibers.
Canadian Patent 841,368 discloses a cigarette filter made from acrylonitrile microfibers. The fibers are made by evaporation of a solvent of an acrylonitrile solution and coagulating the polymer.
U.S. Pat. Nos. 3,444,863 and 3,704,192 disclose spray spun filaments which are useful as cigarette filters. These filaments have diameters ranging from 10 to 60 microns and rely on the large diameter variations in the filaments to achieve air permeability.
U.S. Pat. No. 4,579,130 discloses a molten blend of two thermoplastics are extruded into fibers which are then twisted and/or crimped. The twisted fibers are then formed into a filter rod.
European Patent Application No. 88103071.2 (Publication No. 0330709) and Taiwan Patent Application No. 77106568 each discloses the use of meltblown web as a cigarette filter. The meltblown web is not drawn under heated conditions in the conversion from web to rod, but is merely pulled through a garniture.
U.S. Pat. No. 4,189,511 discloses a filter which utilizes a fragment of ruptured film in a fibrous matrix to achieve the balance between pressure drop and filterability.
U.S. Pat. No. 5,025,815 discloses a cigarette filter tow which includes fibrillating the tow to achieve filter bulking.
Many of the above patents, particularly U.S. Pat. No. 4,189,511, address the problem of balanced filterability and pressure drop. For example, a very tight filter will provide effective filtration at relatively high pressure drop, making it difficult for the smoker to suck the smoke therethrough. Thus, a cigarette filter must exhibit effective filterability at relatively low pressure drop.